Control apparatus of electric power steering apparatus

ABSTRACT

A control apparatus of an electric power steering apparatus capable of firmly detecting an abnormality of a torque detection value, caused by a drift occurred in a torque sensor and by a connector contact resistor, with a simple arrangement. A current supplied from a power supply flows through a potentiometer (3A) of a torque sensor (3), this current is supplied to a current detecting resistor (13g), a drift occurred in the torque sensor (3) is detected by a voltage across terminals of this resistor, the detected drift detection voltage V is  is read in a microcomputer (21), and the drift detection voltage V is  is compared with a set value to judge as to whether or not the drift is present within a normal range. The torque sensor is constructed of a main potentiometer (3 M ) and a sub-potentiometer (3 S ). A change amount ΔV M  is calculated from a difference between a presently detected value and a previously detected value of the torque detection voltage V 2M  of the main potentiometer. Also, a change amount ΔV S  as to the sub-potentiometer is similarly calculated. An abnormality of the torque detection value is judged in response to these change amounts ΔV M  and ΔV S .

RELATED APPLICATION

This application is a National Stage of International Application No.PCT/JP95/02098 under 35 U.S.C. §371, filed Oct. 13, 1995.

TECHNICAL FIELD

The present invention relates to a control apparatus of an electricpower steering apparatus equipped with an apparatus capable of detectingabnormality of a torque detection value.

BACKGROUND ART

In general, in a control apparatus of an electric power steeringapparatus, a detection is made by a torque sensor constructed by asteering torque potentiometer of a steering system, and an electricmotor for producing steering auxiliary force to the above-describedsteering system is controlled by a control means in response to a torquedetection value of the torque sensor.

As described above, in the control apparatus of the electric powersteering apparatus, since the steering auxiliary force produced by theelectric motor is controlled based on the torque detection value of thetorque sensor, when the torque detection value detected by the torquesensor is brought into the abnormality condition, the correct controlcan be no longer performed. Therefore, it is required to detect theabnormality of the torque sensor.

Here, as the abnormality of the torque sensor, there are two types ofabnormality. That is, the torque detection value is drifted due tovariations in power supply voltages and the aging change in contactresistance values of a connector connected to the torque sensor. Also,the torque detection value becomes abnormal, because of abnormality inthe torque sensor caused by variations in the torque detection value bythe aging change in the contact resistance values of the connector, andalso loose contacts of the sliding contactor of the potentiometer.

Conventionally, as the drift detecting circuit for detecting driftabnormality of the torque sensor, for instance, one drift detectingcircuit as shown in FIG. 20 has been proposed.

In this prior art, a torque sensor 101 is so arranged that appliedsteering torque is converted into torsion angle displacement of atorsion bar, and this torsion angle displacement is detected by a mainpotentiometer 102 and a sub-potentiometer 103 series-connected to themain potentiometer 102. Both end portions of the series circuitconstructed of the series-connected potentiometers 102 and 103 arecommonly connected to a power supply E, and the connection portion ofthe serial circuit is grounded. Torque voltages are derived from slidingcontactors 102a and 103a of the respective potentiometers 102 and 103 bysupplying the current of the power supply thereto, and these torquevoltages are inputted to an electronic control circuit 104 employed in apower steering apparatus. Then, each of torque voltages Vm and Vsappearing at each of input resistors Rm and Rs provided at the inputterminal of this electric control circuit 104 is entered via A/Dconverters 105 and 106 to a microcomputer 107. In the microcomputer 107,a calculation is made of a motor current instruction value based uponthe torque voltage value Vm of the main potentiometer 102, and a driftis detected based upon the torque voltage values Vm and Vs of both ofthe main potentiometer 102 and the sub-potentiometer 103.

As indicated by a solid line in FIG. 21, torque voltages representativeof a mutual reverse phase characteristic (cross characteristic) areproduced from the respective potentiometers 102 and 103 constructed inthe above manner. The torque voltage Vm of the sliding contactor 102aand the torque voltage Vs of the sliding contactor 103a become the samevalue as the voltage value Vo when the input torque is zero. Forinstance, assuming now that the respective sliding contactors 102a and103a are moved together toward the lower side in the circuit diagram ofFIG. 20 by applying right steering torque thereto, the torque voltage Vmis decreased in a substantially linear form, whereas the torque voltageVs is increased in a substantially linear form. On the other hand,assuming now that the respective sliding contactors 102a and 103a aremoved together toward the upper side by applying left steering torquethereto, the torque voltage Vm is increased in a substantially linearform, whereas the torque voltage Vs is decreased in a substantiallylinear form. Then, the torque voltages Vm and Vs represent the samevoltage values when the absolute values of the applied torque are equalto each other.

In the torque sensor having such an output characteristic, a drift intorque sensor outputs caused by the variations in the power supplyvoltage, and also the aging changes in the contact resistance values ofthe connector connected to this torque sensor is detected as follows:

For example, assuming now that the power supply voltage E applied to thetorque sensor 101 is decreased due to the temperature changes, driftsappearing in the respective torque voltages Vm and Vs have small amountswhen the output voltage is low, whereas drifts own large mounts when theoutput voltage is high, as indicated by a broken line of FIG. 21, namelythese drifts are not constant. Therefore, first of all, a voltage Voappearing at a neutral point, corresponding to an average value of thetorque voltage Vm and the torque voltage Vs when there is no drift, ispreviously precalculated based on the following formula:

    Vo=1/2(Vm+Vs)                                              (1).

Then, with employment of the torque voltage Vmd of the mainpotentiometer containing the drift and the torque voltage Vsd of thesub-potentiometer containing the drift, a drift value ΔVd is calculatedas an average value of deviation with respect to the voltage Voappearing at the neutral point based on the following formula:

    ΔVd=1/2(Vmd+Vsd)-Vo                                  (2).

Next, an absolute value of this drift value ΔVd is calculated. Thiscalculated absolute value is compared with a preset value. Then, anoccurrence of this drift is detected by judging whether or not thecalculated value is larger than the preset value.

However, in the above-described conventional drift detection circuit,the drift is detected based upon a difference between the torquevoltages of the two systems constructed of the main potentiometer andthe sub-potentiometer. The drift value "Vd" is given from theabove-described formulae (1) and (2) as follows:

    ΔVd=1/2(Vmd-Vm)+1/2(Vsd-Vs)                          (3).

As a consequence, the drift value ΔVd is defined by adding a half of thedeviation component of the main potentiometer to a half of the deviationamount of the sub-potentiometer. For example, since the deviation amountof the sub-potentiometer becomes substantially zero at the left end ofthe characteristic diagram shown in FIG. 21, the drift value ΔVd to becalculated becomes only the 1/2 deviation amount of the mainpotentiometer.

As described above, since the drift calculated in the above-describedprior art is detected as such a value smaller than the actual variationvalue of the power supply voltage, the drift detection sensitivity wouldbe lowered. There is a problem that the drift could not be detecteduntil the difference in the output signal voltages of the two signalpaths becomes a certain large value. As a consequence, there is a riskthat the drive current containing the drifts will flow through theelectric motor until the drifts are detected, and thus the steeringwheel would be self-steered. This may impede safety drive.

On the other hand, the torque-detection-value-abnormality detectingapparatus for detecting the abnormality of the torque detected valuefrom the torque sensor is described in, for instance, Japanese Patentpublication No. Hei. 6-9973. In this prior art, when the differencebetween the torque detected values outputted from the firstdisplacement-to-electric signal converting unit and the seconddisplacement-to-electric signal converting unit employed in the torquesensor is larger than or equal to a predetermined value, a judgement ismade that the torque sensor is abnormal. At this time, the operations ofthe electric motor and the electromagnetic clutch of the electric powersteering apparatus are stopped so as to maintain the vehicle under safestate.

However, in the above-explained conventionaltorque-detection-value-abnormality detecting apparatus, the differencevalue between the main torque detection value and the sub-torquedetection value is compared with a preset value to judge theabnormality. When this predetermined value ΔT is preset, in order tocorrectly judge the abnormality, it is desirable to preset the value byconsidering tolerance in the connector contact resistance valuespredictable during the manufacturing/assembling operations. Thus, forexample, as shown in FIG. 22, in the case that there is the toleranceT_(OFF) in the second displacement-to-electric signal converting unit onthe sub-signal path side, since a larger preset value ΔT is set bytaking this tolerance T_(OFF) into consideration and also by giving aclearance so as to stably judge the normal/abnormal conditions, there isa problem that precision in detecting the abnormality would be lowered.

Also, in the prior art described in the above-mentioned publication, asindicated in FIG. 23 for instance, when the connector contact resistancevalue of the signal line in the first displacement-to-electric signalconverting unit on the main signal path side is increased to therebyvary the torque voltage value, the variation range (i.e., difference intorque detection values) near the neutral position where the appliedsteering torque is low would appear as a small value, as compared withthe variation range where, for instance, the right steering torque ishigh. As described above, there are differences in the variation rangesevery time the steering torque is applied. Thus, there is anotherproblem. That is, it is difficult to keep the detection precisionconstant.

DISCLOSURE OF THE INVENTION

The present invention is intended to solve the above-described problems,and has an object to provide a control apparatus of an electric powersteering apparats with a simple structure, capable of firmly detecting adrift contained in a torque sensor output, and also capable of avoidinga self-steer of a steering wheel.

Further, the present invention has another object to provide a controlapparatus of an electric power steering apparatus capable of improvingdetection precision of abnormality in a torque detection value, and alsocapable of stabilizing the torque detection precision.

To achieve this object, a control apparatus of an electric powersteering apparatus, is characterized as including a torque sensor fordetecting steering torque of a steering system and equipped with atorque detecting potentiometer having a resistance body, both ends ofwhich being connected to a power supply, and a sliding contactor slid onthe resistance body for outputting a torque detection value; an electricmotor for producing a steering auxiliary force to the steering system;control means for outputting a control signal used to control theelectric motor in response to at least the torque detection valueoutputted from the torque sensor; and drive means for driving theelectric motor in response to the control signal of the control means,the control apparatus comprises current detecting means for detecting apower supply current flowing between both ends of the potentiometer; anddrift detecting means for comparing the output value of the currentdetecting means with a preset value to thereby detect a drift occurredin the potentiometer.

According to one embodiment the invention, the current supplied from thepower supply flows through the torque detecting potentiometer, thiscurrent is detected by the current detecting means, and the detectedvalue is compared with a preset value by the drift detecting means todetect the drift occurred in the potentiometer. The drifts caused by thevariation of the power supply and the changes in the connection resistorof the connector, is detected as a change in the currents. The currentdetecting means is constructed of, for example, a fixed resistor. Whenthe current is entered via the potentiometer to this fixed resistor, thedrift appears as a change in the terminal voltage of this fixedresistor. This terminal voltage value is compared with the preset valueby the drift detecting means, so that the drift occurred in thepotentiometer can be firmly detected.

A control apparatus of an electric power steering apparatus, accordingto one aspect of this embodiment of the invention, is characterized inthat a torque detection value setting means for setting the torquedetection value supplied when no steering torque is applied to the samevalue as the voltage value outputted from the potentiometer to therebyoutput the set torque detection value to the control means, is arrangedbetween the same power supply and ground as those of the potentiometer.

In accordance with this aspect of the invention when no steering torqueis applied, the torque detection value supplied from the torquedetecting potentiometer to the torque detection value setting means isset to be equal to the voltage value outputted from the potentiometer,and then the resultant value is outputted to the control means, forexample, even when the drifts caused by the increase in the connectorcontact resistance value of the torque signal input/output terminalhappens to occur.

Furthermore, a control apparatus of an electric power steeringapparatus, according to an additional aspect of the invention, ischaracterized in that the torque sensor is constructed by a currentinput terminal and a drift output terminal connected to both ends of thepower supply; a potentiometer connected between both of the currentinput terminal and the drift output terminal; and a torque signal outputterminal connected to the sliding contactor of the potentiometer, andthat the torque detection value setting means is constructed of torquedetection value setting resistors each connected between the torquesignal output terminal and both ends of the power supply.

According to the invention as defined in claim 3, even when the driftcaused by the increases in the connector contact resistor at the torquesignal output terminal and the like happens to occur, the torquedetection value when no steering torque is applied is set to be equal tothe voltage value outputted to the potentiometer by way of the torquedetection value setting resistor, which will then be outputted to thecontrol means.

Also, a control apparatus of an electric power steering apparatus,according to yet another aspect of the present invention, ischaracterized in that, in a control apparatus of an electric powersteering apparatus including a torque sensor for detecting steeringtorque of a steering system and equipped with a torque detectingpotentiometer having a resistance body, both ends of which beingconnected to a power supply, and a sliding contactor slid on theresistance body for outputting a torque detection value; an electricmotor for producing a steering auxiliary force to the steering system;control means for controling the steering auxiliary force generated bythe electric motor in response to at least the torque detection valueoutputted from the torque sensor: the torque sensor is constructed of amain potentiometer and a sub-potentiometer; the control means controlsthe steering auxiliary force generated by the electric motor based uponat least torque detection value of the main potentiometer; and thecontrol apparatus further comprises abnormality detecting means forcalculating a change amount per unit hour at the same time as to each oftorque detection values of the main potentiometer and of thesub-potentiometer, and also for calculating a difference value betweenthe calculated change amount of the main potentiometer and thecalculated change amount of the sub-potentiometer to thereby detectabnormality of the torque detection value based on the difference value.

In accordance with this aspect the invention, a calculation is made ofthe difference value at the same time between the change amount of thetorque detection value per unit hour, outputted from the main torquesensor, and the change amount of the torque detection value per unithour, outputted from the sub-potentiometer. From this difference value,the abnormality of the torque detection value is detected. For instance,the abnormalitys of the torque detection value are detected, which are,for example, the malfunction of the torque sensor caused by variationsin the torque detection values due to the aging effects of the connectorcontact resistance values, and also by the loose contacts of the slidingcontactors of the potentiometers. Moreover, as to this abnormalitydetection of the torque detection value, the abnormality is not merelydetected from the absolute difference value of the torque detectionvalues of the main torque sensor and the sub-torque sensor. But, theabnormality is detected based upon the relative difference between thechange amount of the torque detection value per unit hour, outputtedfrom the main torque sensor, and the change amount of the torquedetection value per unit hour, outputted from the sub-torque sensor.Accordingly, it is possible to obtain the stable difference value of thetorque detection values irrelevant to the magnitudes of the appliedsteering torque. As a consequence, even when a very small steering forceis applied, i.e., a very small torque detection value, the abnormalityof the torque detection value can be detected in a similar manner when alarge steering force is applied.

A control apparatus of an electric power steering apparatus, accordingto this aspect of the invention, is further characterized in that thetorque sensor is constructed by connection terminals connected to bothends of the power supply; the main potentiometer/sub-potentiometerconnected in parallel between the connection terminals; and a torquedetection value output terminal connected to sliding contactors of bothof the main and sub-potentiometers.

According to this further characterized aspect of the invention, theexternal connection terminal of the torque sensor can be constructed ofthree components, i.e., one pair of connection terminals and the torquesignal output terminal. The structure of the external connectionterminal can be made simple, and can be easily connected to the controlmeans.

A control apparatus of an electric power steering apparatus, accordingto the invention, may also be characterized in that the torque sensor isconstructed by two sets of connection terminals connected in parallelbetween both ends of the power supply; a main potentiometer connected toone set of connection terminals; a sub-potentiometer connected to theother set of connection terminals; and torque detection value outputterminals connected to sliding contactors of both of the main andsub-potentiometers.

In accordance with this characterization of the invention, it is alsopossible to detect the increase in the contact resistance value occurredin the power supply line. The changes in all of the contact resistancevalues existing in the torque sensor and the control means can bedetected, and the abnormality of the torque detection values caused bythis can be detected.

Furthermore, a control apparatus of an electric power steeringapparatus, according to yet another aspect of the present invention, ischaracterized in that, in a control apparatus of an electric powersteering apparatus including a torque sensor for detecting steeringtorque of a steering system and equipped with a torque detectingpotentiometer having a resistance body, both ends of which beingconnected to a power supply, and a sliding contactor slid on theresistance body for outputting a torque detection value; an electricmotor for producing a steering auxiliary force to the steering system;control means for outputting a control signal used to control theelectric motor in response to at least the torque detection valueoutputted from the torque sensor; and drive means for driving theelectric motor in response to the control signal of the control means:the torque sensor is constructed of a main potentiometer and asub-potentiometer; the control means controls the steering auxiliaryforce generated by the electric motor based upon at least torquedetection value of the main potentiometer; and the control apparatusfurther comprises abnormality detecting means for calculating a changeamount per unit hour at the same time as to each of torque detectionvalues of the main potentiometer and of the sub-potentiometer, and alsofor calculating a difference value between the calculated change amountof the main potentiometer and the calculated change amount of thesub-potentiometer to thereby detect abnormality of the torque detectionvalue based on the difference value; current detecting means fordetecting a power supply current flowing between both the ends of thepotentiometer; and drift detecting means for comparing the output valueof the current detecting means with a preset value to thereby detect adrift occurred in the potentiometer.

According to this aspect of the invention, the drifts produced in thepotentiometers can be firmly detected. Also, for example, theabnormality of the torque detection values such as the malfunction ofthe torque sensor can be correctly detected, which are caused by thevariation in the torque detection value due to the aging effect of theconnector contact resistance value, and by the loose contacts of thesliding contactors of the potentiometers irrelevant to such a very smallsteering force under which the torque detection value is low, and such alarge steering force.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a claim corresponding block diagram according to claim 1 ofthe present invention.

FIG. 2 is a claim corresponding block diagram according to claim 2 ofthe present invention.

FIG. 3 is a claim corresponding block diagram according to claim 3 ofthe present invention.

FIG. 4 is a schematic structural diagram for showing one example of anelectric power steering apparatus according to the present invention.

FIG. 5 is a block diagram for representing such a case that a driftabnormality of a torque detection value is detected according to a firstembodiment of the present invention.

FIG. 6 is a characteristic diagram for indicating a drift characteristicaccording to the first embodiment.

FIG. 7 is a characteristic diagram for showing a torque detectionvoltage according to the first embodiment.

FIG. 8 is a flow chart for indicating a process sequence to detect thedrift according to the first embodiment.

FIG. 9 is a flow chart for representing a control process sequenceexecuted by a central processing unit during normal operation of theelectric power steering apparatus, according to the first embodiment.

FIG. 10 is a characteristic diagram for showing a motor currentinstruction value with respect to steering torque while a vehicle speedis used as a parameter.

FIG. 11 is a flow chart for indicating the abnormality monitoringprocess sequence of a motor drive current according to the firstembodiment.

FIG. 12 is a block diagram for showing such a case that the presentinvention is applied so as to detect an abnormal torque detection value,according to a second embodiment.

FIG. 13 is a characteristic diagram for denoting an input/outputcharacteristic of a potentiometer according to the second embodiment.

FIG. 14 is an equivalent circuit diagram containing a contact resistanceR_(t).

FIG. 15 is a characteristic diagram for indicating a change in a torquedetection voltage when the contact resistance is increased.

FIG. 16 is a flow chart for indicating the abnormality detecting processof the torque detection value.

FIG. 17 is a block diagram for indicating such a case that the presentinvention is applied so as to detect an abnormal torque detection value,according to a third embodiment.

FIG. 18 is an equivalent circuit diagram containing a contact resistanceR_(c).

FIG. 19 is a schematic block diagram for showing a control circuit forsensing a steering condition based upon the torque detection value,steering angle speed value, and steering angle acceleration value.

FIG. 20 shows a conventional torque detecting circuit diagram includingdrift detection.

FIG. 21 is a characteristic diagram for indicating the driftcharacteristic according to the prior art.

FIG. 22 is an explanatory diagram for explaining conventional torquevoltage offset.

FIG. 23 is an explanatory diagram for representing a change in changingwidths of torque voltages.

BEST MODE TO PRACTICE THE INVENTION

Referring now to drawings, an embodiment of the present invention willbe explained.

FIG. 4 is a schematic structural diagram of an electric power steeringapparatus according to the present invention. In this drawing, referencenumeral 1 is a steering wheel. The steering force effected to thissteering wheel is transferred to a steering shaft 2 constructed of aninput shaft 2a and an output shaft 2b. One end of this input shaft 2a iscoupled to the steering wheel 1, and the other end thereof is coupled toone end of the output shaft 2b via a torque sensor 3 functioning as atorque detecting means.

Then, the steering force transferred to the output shaft 2b istransferred via a universal joint 4 to a lower shaft 5, and is furthertransferred via a universal joint 6 to a pinion shaft 7. Furthermore,the steering force is transferred via a steering gear 8 to a tie rod 9so as to steer a steering wheel. The steering gear 8 is arranged in arack-and-pinion form containing a pinion 8a and a rack 8b. The rotarymoment transferred to the pinion 8a is translated into the linearpropagation moment by way of the rack 8b.

Reduction gears 10 for transferring auxiliary steering force (assistforce) to the output shaft 2b is coupled to the output shaft 2b of thesteering shaft 2, whereas an output shaft of an electric motor 12 forgenerating the auxiliary steering force is coupled to the reductiongears 10 via an electromagnetic clutch apparatus 11 constructed of, forexample, an electromagnetic type clutch apparatus fortransferring/interrupting the auxiliary steering force.

This electromagnetic clutch apparatus 11 includes a solenoid. Thereduction gears 10 are mechanically coupled to the electric motor 12 bysupplying an energizing current to this solenoid by a controller 13(which will be discussed later), and are mechanically decoupled from theelectric motor 12 by stopping the supply of the energizing current.

A torque sensor 3 senses steering torque which is applied to thesteering wheel 1 and then transferred to the input shaft 2a. The torquesensor 3 is so arranged that, for example, the steering torque istranslated into torsion angle displacement of a torsion bar interposedbetween the input shaft 2a and the output shaft 2b, this torsion angledisplacement is translated into linear propagation moment by a ballscrew member, and a lever coupled to a sliding contactor of apotentiometer is moved by the ball screw member, and then a resistancevalue outputted from the sliding contactor is variable. A torquedetection value constructed of a voltage value outputted from thesliding contactor of this torque sensor 3 is supplied to a controller13.

In response to the torque detection value T and the current detectionvalue of the electric motor 12, the controller 13 controls the drivecurrent supplied to the electric motor 12. A signal corresponding t thedrive current of the electric motor 12 is fed back to this controller13, and in response to this feed back signal, the current outputted fromthe controller 13 to the electric motor 12 is feed-back controlled.Furthermore, a current is supplied from a battery 16 through an ignitionswitch 14 and a fuse 15a. In addition, a vehicle speed detection signalV_(P) from a vehicle speed sensor 17 for producing a pulse signal havinga time period corresponding to, for example, an output speed of agearbox (transmission) is entered into the controller 13 to therebyproduce auxiliary steering force in response to the vehicle speed.

In FIG. 5, there is shown a basic arrangement of the torque sensor 3 andthe controller 13, according to an embodiment of the present invention.As shown in this drawing, a potentiometer 3A of the torque sensor 3 isso constructed as to derive a variable resistance value from a slidingcontactor 3d, and has a resistance value R_(t), across the both endsthereof. Both of the end portions of the potentiometer 3A are connectedto a current input terminal 3a and a drift output terminal 3c of thetorque sensor 3, and a sliding contactor 3d for outputting the torquedetection value T is connected to the torque signal output terminal 3b.

The respective terminals 3a to 3c of the torque sensor 3 are connectedto a cable 26 having connectors at both ends thereof, and are connectedvia this cable 26 to a current output terminal 13a, a torque signalinput terminal 13b, which are provided with the controller 13,respectively.

Here, it is assumed that a connector contact resistance value of thecurrent output terminal 13a is "R1", the respective connector resistancevalues of the current input terminal 3a, drift output terminal 3c anddrift input terminal 13c are "R2", "R3" and "R4", and connector contactresistance values of the torque signal output terminal 3b and the torquesignal input terminal 13b are "R5" and "R6", which connected by theconnectors.

A power voltage E is applied from a stabilizing power supply circuit VRvia a current detection resistor 13d having a resistance value Ris tothe current output terminal 13a. Then, the torque signal input terminal13b is connected to a junction point between torque detection valuesetting resistors 13e and 13f series-connected to each other,functioning as a torque detection value setting means. This junctionpoint is connected to a phase compensating circuit 18 (which will beexplained later). Both of the torque detection value setting resistors13e and 13f each own a resistance value Rtm. The current supplied fromthe stabilizing power supply circuit VR will flow through the torquedetection value setting resistors 13e and 13f to the ground potential.Then, the drift input terminal 13c is grounded via a current detectingresistor 13g functioning as a current detecting means having theresistance value Ris. A power supply is supplied via the ignition switch14 and the fuse 15a from the battery 16 to the stabilizing power supplycircuit VR.

The torque detection value T inputted to the torque signal inputterminal 13b is supplied to the phase compensating circuit 18 forcompensating for the phase of the inputted signal to thereby increasestability of the electric power steering apparatus.

A drift detection voltage Vis appears across the current detectionresistor 13g provided with the drift input terminal 13c, and thisvoltage is supplied to an A/D converter 20b.

In this case, an input impedance of the phase compensating circuit 18 isset so as to have a sufficiently large value, as compared with theresistance value Rt of the potentiometer 3A, the resistance value Rtm ofthe respective torque detection value setting resistors 13e and 13f, andalso the resistance value R is of the current detection resistor 13g. Asa consequence, the influence given by the input impedance of the phasecompensating circuit 18 can be substantially neglected.

The controller 13 includes the torque detection value setting resistors13e, 13f, the current detection resistor 13g, the phase compensatingcircuit 18, and a stabilized power supply circuit VR. Further, thecontroller 13 is constructed of a counter circuit 19 for accumulatingthe pulse number of the vehicle speed detecting signal Vp per unit hour,supplied from the vehicle speed sensor 17, to output the vehicle speeddetection value V, and also whose count value is reset by amicrocomputer 21 (will be described later) when the vehicle speeddetection value V is read into the microcomputer 21. The controller 13further includes A/D converters 20a to 20d which convert an outputsignal Tp of the phase compensating circuit 18, the drift detectionvoltage Vis, and motor current detection signals I_(R), I_(L) (will bediscussed later) into a torque detection value T_(D), a drift detectionvalue Vid, and a motor current detection value i_(R), i_(L) in digitalsignal forms, respectively. The controller 13 is furthermore arranged bythe microcomputer 21 to which the output signals from the countercircuit 19 and the A/D converters 20a to 20d are supplied; a motor drivecircuit 22 functioning as a drive means for driving the electric motor12 in response to the output signal from the microcomputer 21; afail-safe relay circuit 23 functioning as a fail-safe means forsupplying an entered power supply current to the motor drive circuit 22;a clutch drive circuit 24 for driving the electromagnetic clutchapparatus 11 in response to the output signal from the microcomputer 21;and also a current detecting circuit 25 for detecting a magnitude and adirection of the motor current and for feeding back motor currentdetection signals I_(R), I_(L) thereof to the microcomputer 21.

The microcomputer 21 includes an input interface 21a into which theoutput signals from the counter circuit 19 and the A/D converters 20a to20d; a central processing unit (CPU) 21b for executing a drift detectingprocess functioning as a drift detecting means, and a drive controlprocess of the electric motor 12 in response to the torque detectionvalue T; a memory 21c for storing therein a set value used tocompare/detect the drifts, a process sequence for performing a controlafter the drift detection, and a process sequence for controlling thedrive of the electric motor 12; and also an output interface 21d.

From this output interface 21d, a pulse width modulation signal PWM, aright direction signal D_(R), and a left direction signal D_(L) areoutputted. The pulse width of this pulse width modulation signal PWM ischanged in response to a voltage value of a motor drive signal S_(M)(will be discussed later) outputted from the central processing unit21b. The right direction signal D_(R) and the left direction signalD_(L) define the rotation directions of the electric motor 12. Thesesignals are supplied to the motor drive circuit 22. Further, a relaycontrol signal S_(R) and a clutch control signal, S_(C) are outputtedfrom the output interface 21d, and are furnished to the fail-safe relaycircuit 23 and the clutch drive circuit 24.

The motor drive circuit 22 owns a gate drive circuit 22a, an H bridgecircuit 22b, and a voltage boosting (step-up) power supply 22c.

The gate drive circuit 22a outputs the supplied right/left directionsignals D_(R), D_(L) to the H bridge circuit 22b, and also shapes thewaveform of the supplied pulse width modulation signal PWM in order toimprove the response characteristic of the electric motor 12. Thewaveform-shaped pulse width modulation signal PWM is outputted to the Hbridge circuit 22b. In response to the supplied right/left directionsignals D_(R), D_(L), the gate drive circuit 22a switches the pulsewidth modulation signal PWM to be outputted. For instance, when theright direction signal D_(R) is supplied, the pulse width modulationsignal PWM is outputted only to an FET (field-effect transistor) 22b2(will be explained later) of the H-bridge circuit 22b. When the leftdirection signal D_(L) is supplied, the pulse width modulation signalPWM is outputted only to an FET 22b1 (will be described later).

The H bridge circuit 22b supplies the drive current to the electricmotor 12 based upon the output signal of the gate drive circuit 22a, andincludes four switching transistors, for example, N-channel power MOStype FETs 22b1 to 22b4. A series circuit to connect the source terminalof the FET 22b1 with the drain terminal of the FET 22b3 is provided inparallel to a series circuit to connect the source terminal of the FET22b2 with the drain terminal of the FET 22b4, and the electric motor 12is interposed between the connected portions of the FETs in therespective series circuit. The pulse width modulation signal PWM issupplied from the gate drive circuit 22a to the gate terminals of therespective FETs 22b1 and 22b2 at the upper stage in response to theright/left direction signals D_(R), D_(L), whereas the right directionsignal D_(R) and the left direction signal D_(L) are supplied from thegate drive circuit 22a to the gate terminals of the respective FETs 22b3and 22b4 at the lower stage. The battery 16 is connected via the failsafe relay circuit 23, the fuse 15a, and the ignition switch 14 to therespective drain terminals of the FET 22b1 and the FET 22b2. Therespective source terminals of the FET 22b3 and the FET 22b4 aregrounded via the respective current detection resistors R_(R) and R_(L).

In this case, when the right direction signal D_(R) and the leftdirection signal D_(L) are at high levels, the FET 22b3 and the FET 22b4are brought into ON states. When the FET 22b3 is under ON state, thepulse width modulation signal PWM is supplied to the FET 22b2, so thatthe motor drive current flows from the FET 22b2 to the direction of theelectric motor 12 and the FET 22b3. On the other hand, when the FET 22b4is under ON state, since the pulse width modulation signal PWM tosupplied to the FET 22b1 , the motor drive current flows from the FET22b1 to the direction of the electric motor 12 and the FET 22b4.

The voltage boosting power supply 22c is constructed in such a mannerthat for instance, a battery voltage required to drive the FETs 22b1 and22b2 within the H bridge circuit 22b is boosted twice an the boostedbattery voltage is applied to the gate drive circuit 22a. It should benoted that the battery voltage is applied via the fail safe relaycircuit 23ato the gate drive circuit 22a so as to drive the FETs 22b3and 22b4.

The fail safe relay circuit 23 is arranged by a fail safe relay 23ahaving a relay constance 23c, and a relay drive circuit 23b forsupplying an energizing current to a drive coil of the fail safe relay23a. The relay drive circuit 23b is controlled in response to thesupplied relay control signal S_(R). One end of the relay contact 23c isconnected via the fuse 15a and the ignition switch 14 to the battery 16.The other end of this relay contact 23c is connected to the respectivedrain terminals of the FETs 22b1 and 22b2 of the H bridge circuit 22.

In this first embodiment, when the relay control signal S_(R) is at thehigh level, the relay drive circuit 23b is brought into the ON state, sothat the energizing circuit is supplied to the drive coil of the failsafe relay 23a to thereby close the relay contact 23c. On the otherhand, when the relay control signal S_(R) is at the low level, the relaydrive circuit 23b is brought into the OFF state to thereby open therelay contact 23c. In general, the relay contact 23c is closed while theelectric power steering apparatus is operated, and the relay contact 23cis opened when the abnormality happens to occur in the motor drivecircuit 22 and the like so as to keep safety.

The clutch drive circuit 24 amplifies the supplied clutch control signalS_(C) to output such a signal for controlling the drive operation of theelectromagnetic clutch apparatus 11. In this embodiment, when the clutchcontrol signal S_(C) is at the high level, the electromagnetic clutchapparatus 11 is brought into the connect state, whereas when the clutchcontrol signal S_(C) is at the low level, the electromagnetic clutchapparatus 11 is brought into the non-contact state.

The current detecting circuit 25 amplifies inputted voltages appearingat the respective terminals of the current detection resistors R_(R) andR_(L) so as to detect the respective motor drive currents, so that noiseis removed. Thus, the detected motor current detection signal I_(R) inthe right direction and the detected motor current detection signalI_(L) in the left direction are fed back to the input interface 21a viathe respective A/D converters 20c and 20d. As a result, in accordancewith the actually measured values of the motor currents, the pulse widthof the pulse width modulation signal PWM is corrected, and the operationof the electric power steering apparatus during the occurrence ofabnormality is stopped by the microcomputer 21.

In the torque sensor 3 and the controller 13 with the above-describedcircuit arrangement, a current I_(S) will flow through the currentdetection resistor 13d, the potentiometer 3A, and the current detectionresistor 13g by the stabilized power supply circuit VR having the outputvoltage E. This current value I_(S) may be expressed by considering theconnector contact resistance values R1 to R4 as follows:

    I.sub.S =E/ R.sub.t +2R.sub.is +(R1+R2+R3+R4)!             (4)

The current values I_(S) is changed in response to variations of thepower supply voltage E and variations of the connector contactresistance values R1 to R4. Assuming now that the power supply voltage Eis varied and then becomes E±ΔE, the current value I_(S) is given asfollows:

    I.sub.S =(E±ΔE)/ Rt+2R.sub.is +(R1+R2+R3+R4)!     (5)

As a result, the drift detection voltage V_(is) at the terminal of thecurrent detection resistor 13g is given as follows:

    V.sub.is =R.sub.is (E±ΔE)/ Rt+2R.sub.is +(R1+R2+R3+R4)!(6)

As described above, the drift detection voltage V_(is) is varied inresponse to the variation of the power supply voltage E. Consequently,the drifts occurred in the torque sensor 3, which are caused by thevariations of the power supply voltage E and the variations of theconnector contact resistance values R1 to R4 can be detected bydetecting the current flowing through the potentiometer 3A by way of thecurrent detection resistor 13g. The drift detection voltage V_(is) isnot changed by the input torque, as shown in FIG. 6, and then when thedrift is produced, this drift detection voltage V_(is) is uniformlyincreased/decreased in proportion to the drift amount, as indicated by abroken line.

The torque detection value T detected by the potentiometer 3A is derivedas a torque voltage V_(t) by way of the torque detection value settingresistors 13e and 13f of the controller 13. A torque voltage V_(to) whenthe sliding contactor 3d is located at the neutral position may beexpressed as follows:

    V.sub.to =E R.sub.t /2+R.sub.is +(R3+R4)!/ R.sub.t +2R.sub.is +(R1+R2+R3+R4)!                                           (7)

As indicated in FIG. 7, the torque voltage V_(t) is changed in a linearform with respect to the input torque. When the input torque becomeszero, it becomes a voltage V_(to), When the left steering torque isincreased, the voltage value of the torque voltage V_(t) is increased,whereas when the left steering torque is increased, the voltage valuethereof is decreased.

As indicated by a broken line, there is an insensitive band "Δα" nearthe voltage value V_(to) as a torque value width where even when thesteering torque is applied, the electric motor is not driven. A selfsteer of a steering wheel happens to occur in such a case that, forinstance, the neutral position voltage when the input torque becomeszero is changed due to the drift, and exceeds either an upper limitvoltage value VH of the insensitive band Δα, or a lower limit voltagevalue VL thereof, and then the torque voltage reaches a predeterminedvoltage value.

In this first embodiment, when the sliding contactor 3d is located atthe neutral position, since the torque value setting resistors 13e and13f having the same resistance values Rtm are employed, the torquevoltage V_(t) inputted to the phase compensating circuit 18 ismaintained at the same voltage value as the neutral position voltage ofthe sliding contactor 3d. As a consequence, even when such a drifthappens to occur, by which the resistances of the respective connectorcontact resistors R5 and R6 of the torque signal output terminal 3b andthe torque signal input terminal 13b are increased, when the slidingcontactor 3d is located at the neutral position, the torque voltageV_(t) is maintained at a constant value, so that the self steer of thesteering wheel can be avoided.

Next, a process sequence to detect a drift, executed in the centralprocessing unit 21b, according to this embodiment will now be explainedwith reference to a flow chart of FIG. 8.

This process operation is carried out by executing, for instance, atimer interrupt to a predetermined main program every preselected time,e.g., executed every several msec.

First, at a step S1, the drift detection voltage V_(is) is read via theA/D converter 20b. Next, at a step S2, a comparison is made between thedrift detection voltage V_(is), and a minimum value Vmin and a maximumvalue Vmax, which are used to detect the drift and have been previouslyset into the memory 21c.

It should be noted that the minimum value Vmin is set to such a valuethat no self steer happens to occur even when the power supply voltage Eis lowered, and the connector contact resistance values R1 to R4 areincreased. For example, this minimum value Vmin is set to a valueslightly lower than the drift detection voltage V_(is) corresponding tothe lower limit voltage value VL of the insensitive band Δα. Also, themaximum value Vmax is set to such a value that no self steer happen tooccur even when the power supply voltage E is increased. This maximumvalue Vmax is set to, for example, a value slightly higher than thedrift detection voltage V_(is) corresponding to the upper limit voltageVH of the insensitive band Δα. Then, when the drift detection voltageV_(is) is higher than or equal to the minimum value Vmin, and also islower than or equal to the maximum value Vmax, it is judged that thedrift is present within the normal range. Accordingly, the driftdetection is completed, and the process operation is returned to themain program in which the normal motor drive control process shown inFIG. 8 is executed to control the generation of the auxiliary steeringforce. These process operations defined at the steps S1 and S2correspond to the drift detecting means.

In the case that as a judgement result of the step S2, either 0≦V_(is)<V_(min) or V_(max) <V_(is), it is judged that the drift is deviatedfrom the normal range, and then the process operation is advanced to afail safe operation process defined by steps after a step 3a.

First, at a step S3a, the pulse width modulation signal PWM, the rightdirection signal D_(R), and the left direction signal D_(L), which aresupplied to the motor drive circuit 22 are set to low levels, so thatthe signals to the H bridge circuit 22b is interrupted. Subsequently,the process operation is advanced to a step S3b which the clutch controlsignal S_(C) is set to allow level and the resultant clutch controlsignal is outputted, so that the electromagnetic clutch apparatus 11 isbrought into the non-connect condition.

Then, the process operation is advanced to a step S3c at which the relaycontrol signal S_(R) is set to a low level and the low-leveled relaycontrol signal S_(R) is outputted, so that the fail safe relay 23a isopened to interrupt the power supply path from the battery 16 to the Hbridge circuit 22b. Next, at a step S3d, an abnormality detection flagis set to "1", and this abnormality notification is made to the highorder program such as the main process program, and then the processoperation is ended. Thereafter, the motor drive control processoperation is not executed in the high order program.

Next, when the above-described drift detection process operation iscarried out and then the drift is located within the normal range, aprocess sequence of a motor drive control process executed by thecentral processing unit will now be explained based on a flow chartshown in FIG. 9.

Also, this process operation is carried out by timer-interrupting, forexample, a predetermined main program every preselected time, e.g.,executed every several msec.

At a first step S21, the torque voltage value T which has beenphase-compensated by the phase compensator 18 and is derived from thetorque sensor 3 is read via the A/D converter 20a.

Subsequently, the process operation is advanced to a step S22 at which acalculation is made of T=T-V_(o), and an offset process is performed insuch a manner that the torque detection value during the neutralposition becomes zero.

Next, the process operation is advanced to a step S23 at which the countvalue of the counter 19, namely the vehicle speed detection value V isread, a reset signal is outputted to the counter 19 so as to reset thecounter value. Subsequently, the process operation is advanced to stepS24. At this step S24, referring to a characteristic diagram indicativeof a correspondence relationship between steering torque, vehiclespeeds, and motor currents, as shown in FIG. 10, for example, aretrieval is made of such a motor current corresponding to the torquedetection value T and the vehicle speed detection value V, andthereafter this motor current is set as a motor current instructionvalue S₁.

This characteristic diagram indicates a table for representing arelationship among the motor current, the steering torque, and thevehicle speed. This motor current is required to drive the electricmotor 12 which may produce the steering assist force corresponding tothe steering torque inputted into the steering shaft 2. The lower thevehicle speed is decreased, the larger the motor current instructionvalue is increased. Also, the larger the steering torque is increased,the larger the motor current instruction value is increased, and whenthis motor current instruction value exceeds a certain value, thisinstruction value is not increased from the certain value.

Then, the process operation is advanced to step S25, at which the motorcurrent instruction value S₁ is differentially processed to obtain adifferentially processed value "f_(D) ". At the next step S26, both of amotor current detection value i_(R) for the right direction and anothermotor current detection value i_(L) for the left direction are read. Themotor current detection value i_(R) for the right direction is set as apositive value, whereas the motor current detection value i_(L) for theleft direction is set as a negative value. These motor current detectionsignals are summed with each other to calculate a motor currentdetection value "i_(M) ". In other words, it is calculated by i_(M)=i_(R) -i_(L).

It should be understood that the current detection circuit 25 performs asufficient filtering process to the respective signals in order toobtain effective values about the motor current detection values i_(R)and i_(L) for the right/left directions.

Subsequently, the process operation is advanced to step S27, at whichsuch a abnormality monitoring process as shown in a flow chart of FIG.11 is performed.

In this abnormality monitoring process, at first step S27a, a judgementis made as to whether or not an absolute value .linevert split.i_(M).linevert split. of the motor current detection value i_(M) is smallerthan a preset maximum current value I_(MAX), by which it is recognizablethat the motor drive circuit 22 is operated under normal condition. Whenthe absolute value .linevert split.i_(M) .linevert split. is smallerthan the maximum current value I_(MAX), it is judged that the motordrive current is present within the normal range, and thus the processoperation is returned to the motor drive control process program.

On the other hand, as a judgement result of the step S27a, when.linevert split.i.sub._(M) .linevert split.≧I_(MAX), an excessivecurrent flows through the H bridge circuit 22b. Thus, it is judged thata abnormality happens to occur, and then the process operation isadvanced to a step S27b. At this step S27b, the levels of the respectiveinstruction signals S_(M), D_(R), D_(L) supplied to the gate drivecircuit 22a are set to "LOW", so that the supply of the power to the Hbridge circuit is interrupted. Subsequently, the process operation isadvanced to step S27c at which the output of the clutch control signalS_(C) to the clutch drive circuit 24 is stopped, so that theelectromagnetic clutch apparatus 11 is operated so as to bring theoutput shaft of the electric motor 12 and the reduction gears 10 intothe disconnect condition.

Then, the process operation is advanced to step S27d at which the relaycontrol signal to the relay drive circuit 23b is set to "LOW", and thusthe fail safe relay 22a is opened, so that the supply of power from thebattery 16 to the H bridge circuit 22b is interrupted. Next, at stepS27e, for instance, the abnormality is notified to the upper gradeprogram such as the main process program, and the process operation iscompleted. Subsequently, the motor drive control process is no longerexecuted in the high order program.

As a result of the abnormality monitoring process defined at the stepS27, when no abnormality is detected from the motor drive current, theprocess operation is advanced to step S28.

At the step S28, current deviation "e_(M) " is calculated based on adifference between the motor current instruction value S₁ set at thestep S24 and the motor current detection value i_(M) calculated at thestep S26, namely e_(M) =S₁ -i_(M).

Next, at step S29, the current deviation e_(M) is multiplied by apredetermined proportional gain to obtain a proportionally processedvalue "f_(P) ". Further, at step S30, this proportionally processedvalue "f_(P) " is integrated to obtain an integrally processed value f₁.Both of the proportionally processed value f_(P) and the integrallyprocessed value f₁ are stored into a predetermined storage region of thememory 21c.

Then, at step S31, the differentially processed value f_(D), theproportionally processed value f_(P), and the integrally processed valuef₁ are added to each other, and the added value is recognized as themotor drive signal S_(M), and then the process operation is advanced tostep S32.

At this step S32, a check is done as to whether or not the motor drivesignal S_(M) is S_(M) ≧0. In the case of S_(M) ≧0, it is recognized thatthe steering wheel 1 is steered along the right steering direction, andthus, the process operation is advanced to step S33. At this step S33,the right direction signal D_(R) for setting the rotation direction ofthe electric motor 12 to the normal rotation direction is set to "HIGH",and the left direction signal D_(L) is set to "LOW". The motor drivesignal S_(M) is outputted to the output interface 21d, and then thevoltage of the motor drive signal S_(M) is converted into the pulsewidth modulation signal PWM having a predetermined pulse width based ona sawtooth wave produced within the output interface 21d. This pulsewidth modulation signal PWM is supplied via the gate drive circuit 22ato the H bridge circuit 22b. Then, the motor drive control processprogram is ended and the process operation is returned to the mainprogram.

On the other hand, when it is not equal to S_(M) ≧0 at the step S32, itis judged that the steering wheel 1 is steered along the left steeringdirection, and the process operation is advanced to step S34. At thisstep S34, the left direction signal D_(C) used to set the rotationdirection of the electric motor 12 along the reverse rotation directionis set to "HIGH", and the right direction signal D_(R) is set to "LOW"which is outputted. Also, the motor drive signal S_(M) is converted intothe pulse width modulation signal PWM which will then be supplied to theH bridge circuit 22b via the gate drive circuit 22a. Then, the motordrive control process program is accomplished and the process operationis returned to the main program.

Next, operations of the above-described first embodiment will now beexplained. In this first embodiment, the drift occurred in the torquesensor is detected every time before the normal torque drive controlprocess is executed. When the drift detection voltage V_(is) is equal toV_(min) ≦V_(is) ≦V_(max), it is so judged that this drift detectionvoltage V_(is) is present within the normal range, and no such a driftcapable of self-steering the steering wheel is produced. Then, theprocess operation is advanced to the normal torque drive controlprocess.

On the other hand, when either 0≦V_(is) <V_(min), or V_(max) <V_(is), itis so judged that the drift is deviated from the normal range, and thusall of the control signals supplied to the H bridge circuit 22b are setto low levels. Furthermore, such a fail safe process is carried out thatthe electromagnetic clutch apparatus 11 is brought into the non-connectstate, the fail safe relay 23a is opened, and the operation of theelectric motor is stopped. As a consequence, the self steer of thesteering wheel is prevented to maintain the safety operation.

When no steering torque is applied and the sliding contractor 3d islocated at the neutral position, the torque detection value T suppliedto the controller 13 becomes the same voltage value as the voltage whenthe sliding contractor 3d is located at the neutral position by theeffects of the torque detection value setting resistors 13e and 13f. Asa consequence, at this time, even when the resistance values of theconnector contact resistances R5 and R6 of the respective input/outputterminals 3b and 13b for the torque signals are increased and a drifthappens to occur, since the torque voltage Vt is kept at the constantvalue, the self steer of the steering wheel can be avoided.

It should be noted that although the current supplied via thepotentiometer is entered into the fixed resistor as the currentdetection resistor so as to detect the current in the above-explainedfirst embodiment, the present invention is not limited thereto.Alternatively, an operational amplifier may be employed instead of thefixed resistor, and a change in the currents may be converted into avoltage to detect the drift. Also, the drift is detected at theterminals of the current detection resistor 13g. Alternatively, evenwhen the drift is detected at the current output terminal 13a of thecurrent detection resistor 13d connected to the power supply, the driftmay be detected by monitoring the current.

In the above-explained first embodiment, although the torque detectionvalue setting resistors 13e and 13f have the same resistance values, thepresent invention is not limited thereto. For instance, the torquedetection value setting registers 13e and 13f having differentresistance values from each other may be arranged in such a manner thatthis output voltage has the same voltage as the torque detection voltageapplied to the phase compensating circuit in response to the outputvoltage of the potentiometer when the steering torque becomes zero. Thismay be similarly applied to the torque detection value setting resistors13e₁, 13f₁, 13e₂, and 13f₂.

Although the microcomputer is employed to judge whether or not the driftamount is present within a preselected range in the above-explainedembodiment, such a comparator as a window comparator may be employedinstead of this microcomputer to compare/detect the drift amount.

Also, in accordance with the first embodiment, the current supplied viathe potentiometer is once converted into a voltage to compare thedetected voltage value with the set voltage value, thereby detection thedrift. Alternatively, the supplied current may be compared with thepreselected constant current by employing a window comparator to therebythe drift.

The process operation to correct the torque detection signal is carriedout by the central processing unit in order to cancel the detected driftamount, and then the electric motor may be driven/controlled byoutputting the corrected motor drive current signal value from thecontroller 13.

Next, a description will be made of a second embodiment in such a casethat an abnormality contained in a torque detection value of a torquesensor is detected with reference to FIG. 12 to FIG. 16.

The second embodiment owns a similar arrangement to that of the firstembodiment except that a torque sensor 3 is arranged by a mainpotentiometer 3_(M) as a main torque sensor and a sub-potentiometer3_(S) as a sub-torque sensor. Torque detection values T and T' areoutputted from both of the potentiometers 3_(M) and 3_(S), and aresupplied to the controller 13, and also a abnormality of the torquedetection value T is detected by the controller 12 based upon the driftdetection process shown in FIG. 8, the motor drive control processindicated in FIG. 9, and the torque detection values T and T'. The samereference numerals shown in FIG. 5 will be employed as those fordenoting the corresponding portions, and detailed descriptions thereofare omitted.

In other words, in the torque sensor 3, the resistance member of themain potentiometer 3_(M) is connected parallel to the resistance memberof the sub-potentiometer 3_(S) to constitute a parallel circuit, one endof the connected end portions is connected to a current input terminal3a, and the other end is connected to a drift output terminal 3c. Eachof the potentiometers 3_(M) and 3_(S) has a resistance value R_(P)across both end portions of the resistance member, and a variableresistance value may be derived from each of sliding contactors 3d and3d' which slide on the respective resistance members, from which thetorque detection values T and T' are outputted. Then, the respectivetorque detection values T and T' is supplied to the respective torquesignal output terminals 3b and 3b'.

In the torque sensor 3 according to this second embodiment, the normalmotor drive control is carried out based upon the torque output value Toutputted from the main potentiometer 3_(M), and a abnormality isdetected in response to the torque detection values T and T' outputtedfrom the main potentiometer 3_(M) and the sub-potentiometer 3_(S). Aninput/output characteristic of this torque sensor 3 is represented inFIG. 13. Both of torque detection voltages V₁ and V₁, functioning as thetorque detection values T and T' of the main potentiometer 3_(M) and thesub-potentiometer 3_(S) are varied in a linear form with respect toinput torque, and become substantially a half of the power supplyvoltage E when the input torque becomes zero, newly, under neutralcondition. While the right steering torque is increased, the voltagevalue is increased, whereas while the left steering torque is increased,the voltage value is decreased. It should be understood that althoughFIG. 13 indicates that the torque detection value V₁ of the mainpotentiometer 3_(M) has a larger value than the torque detection valueV_(1') of the sub-potentiometer 3_(S) with respect to arbitrary inputtorque, there are some possibilities that the torque detection voltageV_(1') of the sub-potentiometer 3_(S) becomes a large value due tofluctuations of parts and assembly.

The respective terminals 3a, 3b, 3b', 3c of the torque sensor 3 areconnected to a cable 26 having connectors at both ends thereof, and areconnected via this cable 26 to the corresponding input terminals, i.e.,a current output terminal 13a, a torque signal input terminal 13b, atorque signal input terminal 13b', and a drift input terminal 13c.

A power supply voltage E is applied from a stabilized power supplycircuit VR via a current detecting resistor 13d having a resistancevalue R_(is) to the current output terminal 13a of the controller 13.Then, the torque signal input terminal 13b is connected to a junctionpoint between the torque detection value setting resistors 13e and 13fseries-connected to each other. Similarly, the torque signal inputterminal 13b' is connected to a joint point between the torque detectionvalue setting resistors 13e' and 13f' series-connected to each other.Each of these joint points is connected to input units of phasecompensating circuit 18 and 18' for compensating a phase of an inputsignal to improve stability of the electric power steering apparatus.The torque detection value setting resistors 13e, 13e' and 13f, 13f'each own a resistance value Rtm. The current supplied from thestabilizing power supply circuit VR will flow through the torquedetection value setting resistors 13e, 13e' and 13f, 13f' to the groundpotential. Then, the drift input terminal 13c is grounded via a currentdetecting resistor 13g having the resistance value R_(is), and isconnected to an A/D converter 20b. A drift detection voltage V_(is)appearing at this current detecting resistor 13g is applied to the A/Dconverter 20b.

In this case, an input impedances of the phase compensating circuits 18,18' and the A/D converter 20b are set so as to have a sufficiently largevalue, as compared with the resistance values R_(P) of thepotentiometers 3_(M), 3_(S), the resistance value Rtm of the respectivetorque detection value setting resistors 13e, 13e' and 13f, 13f', andalso the resistance value R_(is) is of the current detecting resistor13g. As a consequence, the influence given by the input impedances ofthe phase compensating circuit 18, 18' and the A/D converter 20b can besubstantially neglected.

In the microcomputer 21, the central processing unit 21b executes theabove-explained drift detection process and motor drive control process,and also executes the process for detecting the abnormality of 1thetorque detection value as the abnormality detecting means, andfurthermore, the set value for comparing/detecting the change amounts ofthe torque detection values T and T', and the process sequence forperforming the control after detecting the change amount are stored inthe memory 21. Also, a process sequence for driving/controlling theelectric motor 12 is stored into the memory 21.

As to the torque sensor 3 and the controller 13 constructed of theabove-explained structures, among the connector contact resistancevalues R5, R5' and R6, R6' produced in the torque signal outputterminals 3b, 3b' of the torque sensor 3 and the torque signal inputterminals 13b, 13b' of the controller 13, a value obtained by adding theconnector contact resistance values R5 and R6 are used as a contactresistor Rt of a signal line, and an equivalent circuit as to, forexample, the main potentiometer 3_(M) is arranged as shown in FIG. 14.In this FIG. 14, a resistor R₁ indicates a resistance component betweenthe sliding contactor 3d of the main potentiometer 3_(M) and the outputof the stabilized power supply circuit VR, a resistor R₂ shows theresistance value Rtm of the torque detection setting resistor 13e, aresistor R₃ represents a resistance component between the slidingcontactor 3d of the main potentiometer 3_(M) and the ground potential,and a resistor R₄ shows the resistance value Rtm of the torque detectionvalue setting resistor 13f.

In this equivalent circuit, a torque detection voltage V_(2M) enteredinto the phase compensating circuit 18 is expressed by thebelow-mentioned formula (8):

    V.sub.2M = R.sub.4 /(R.sub.2 Rt+R.sub.4 Rt+R.sub.4 R.sub.2)! Rt+R.sub.2 R.sub.3 /(R.sub.1 +R.sub.3)!E                             (8)

In this second embodiment, since the resistance values of the torquedetection value setting resistors 13e and 13f are set to the samevalues, if R₂ =R₄ in the formula (8), then this formula (8) may beexpressed by the following formula (9):

    V.sub.2M = 1/(2Rt+R.sub.2)!  Rt+R.sub.2 R.sub.3 /(R.sub.1 +R.sub.3)!E(9)

At this time, the torque detection voltage V₁ is varied in a linear formwith respect to the input torque as represented in a characteristicdiagram of FIG. 13. When the input torque becomes zero, i.e., at theneutral position, the voltage value appearing on the sliding contactor3d of the main potentiometer 3_(M) is selected to be equal to 1/2voltage value between the power supply and the ground. Accordingly,giving an attention to R₃ /(R₁ +R₃) indicative of a ratio of the torquedetection voltage V₁, assuming now that R₃ /(R₁ +R₃)=α, thecharacteristic diagram of FIG. 13 may be expressed by the followingformula (10):

    (α-1/2)K·T                                  (10)

where symbol "k" denotes a predetermined constant containing the springconstant of the torsion bar, and symbol "T" shows steering torque.

When this formula (10) is substituted for the above-explained formula(9), the following formula (11) is obtained:

    V.sub.2M = R.sub.2 KE/(2Rt+R.sub.2)!T+E/2                  (11)

In accordance with this formula (11), when the input steering torque Tis zero, the torque detection voltage V_(2M) becomes E/2 irrelevant tothe resistance value of the contact resistor Rt produced on the signalline.

Then, when the contact resistor R_(t) is increased, there is a trendthat a ratio of the torque detection voltage V_(2M) to the steeringtorque T is decreased. This is indicated in FIG. 15. A broken line shownin FIG. 15 denotes a characteristic of the main potentiometer 3_(M) whenthe contact resistor R_(t) is an initial value. When the contactresistor R_(t) is increased, the ratio of the torque detection voltageV_(2M) to the steering torque T, namely the inclined amount is decreasedalong the arrow direction. In this second embodiment, based upon theinclined amounts of the main potentiometer 3_(M) and thesub-potentiometer 3_(S), namely the change amounts of the torquedetection voltages V_(2M) and V_(2S) with respect to the input steeringtorque, the abnormality of the torque detection value caused by theincrease of the contact resistor R_(t) is detected.

Then, the process sequence to detect the abnormality of the torquedetection value as the means for detecting the abnormality executed inthe central processing unit will now be described with reference to aflow chart of FIG. 16.

This process operation is executed by interrupting, for example, apredetermined main program every preselected time, e.g., severalmilliseconds.

At first step S1, torque detection voltages V_(2M) and V_(2S) are readvia the A/D converters 20a and 20a'. Then, the process operation isadvanced to step S2, a check is done as to whether or not the readingoperation of the torque detection voltages V_(2M) and V_(2S) correspondsto a first reading operation. In this embodiment, although the torquedetection voltages V_(2M) and V_(2S) are read every time the timerinterrupt is carried out, the difference value is calculated between thepreviously read values and the presently read values in order to detectthe abnormality of the torque detection value. Thus, no calculation iscarried out when the first reading operation is performed. Accordingly,during the first reading operation, the process operation is advanced tostep S2a at which the read torque detection voltages V_(2M) and V_(2S)are stored into a predetermined storage region. Then, the processoperation is returned to the high order program such as the main processprogram. After the second reading operation, the process operation isadvanced to the next step S3.

At this step S3, the torque detection voltage V_(2M) (n-1) stored duringthe previous timer interrupt is subtracted from the presently readtorque detection voltage V_(2M) (n) to thereby calculate a main-sidedtorque voltage change amount ΔV_(M) indicative of a change amount of atorque detection voltage value every preselected time (sampling time) inthe main potentiometer 3_(M). This calculation formula is indicated bythe following formula (12):

    ΔV.sub.M =V.sub.2M (n)-V.sub.2M (n-1)                (12),

where symbol "n" denotes an arbitrary sampling time, and symbol "n-1"represents a sampling time succeeding to a sampling time "n" by 1.

Then, when the calculation is accomplished, the presently read torquedetection voltage V_(2M) is newly stored into a predetermined storageregion where the previously read torque detection voltage V_(2M) hasbeen stored, and the torque detection voltage V_(2M) is updated.

In the case that the steering torque is zero, the main-sided torquevoltage change amount ΔV_(M) is equal to 0, since there is no changeamount in the torque detection voltage V_(2M). At this time, the torquedetection value inputted into the microcomputer 21 is kept at theneutral voltage because of the effects by the torque detection valuesetting resistors 13e, 13f and 13e', 13f'. As a result, when thesteering torque is zero, even if the contact resistor R_(t) isincreased, it can be avoided that the steering shaft is self-steered. Onthe other hand, when the steering torque is slightly varied, it ispossible to obtain the main-sided torque voltage change amount ΔV_(M) incorrespondence with the change amount of the torque detection voltagevalue, so that the main-sided torque voltage change amount ΔV_(M)corresponding to the increase of the contact resistor R_(t) can bedetected. This may be similarly applied to the subsequent sub-sidedtorque voltage change amount ΔV_(S).

Next, the process operation is advanced to a step S4 at which the torquedetection voltage V_(2S) (n-1) stored during the preceding timeinterrupt is subtracted from the presently read torque detection voltageV_(2S) (n) in a similar manner to that of the step S3. Thus, acalculation is made of a sub-sided torque voltage change amount ΔV_(S)indicative of a torque detection voltage change amount in thesub-potentiometer 3_(S) every preselected time. This calculation formulais given by the below-mentioned equation (13):

    ΔV.sub.S =V.sub.2S (n)-V.sub.2S (n-1)                (13)

Then, when the calculation is ended, the present torque detectionvoltage V_(2S) is newly stored into a predetermined storage region wherethe previous torque detection voltage V_(2S) has been stored, and thetorque detection voltage V_(2S) is updated.

Subsequently, the process operation is advanced to a step S5 at whichthe value of the sub-sided torque voltage change amount ΔV_(S) issubtracted from the value of the main-sided torque voltage change amountΔV_(M) and an absolute value of this subtracted value is calculated.Then, a judgement is made as to whether or not this absolute value issmaller than, or equal to a preset value "β" so as to judge whether ornot a difference in the respective torque detection voltages V_(2M) andV_(2S) is located within a range of the preset value "β". This judgementformula is given by (14):

    .linevert split.ΔV.sub.M -ΔV.sub.S .linevert split.≦β                                      (14)

When any one of the contact resistors R_(t) of the main potentiometer3_(M) and the sub-potentiometer 3_(S) is increased, since the values ofthe change amount ΔV_(M) and ΔV_(S) of the potentiometer 3_(M) or 3_(S),whose contact resistor is decreased, a value of .linevert split.ΔV_(M)-ΔV_(S) .linevert split. is increased. As a consequence, the value of.linevert split.ΔV_(M) -ΔV_(S) .linevert split. is compared with thepredetermined value β, so that an increase in the contact resistor R_(t)can be detected. When there is no increase in the contact resistorR_(t), the value of .linevert split.ΔV_(M) -ΔV_(S) .linevert split.becomes zero in principle. However, because of fluctuations in theresistance values of the torque detection value setting resistors 13e,13f and 13e', 13f' used to maintain it at the neutral voltage, and alsodifferences in the resistance values of the initial contact resistorsR_(t) occurred in the signal lines for the respective potentiometers3_(M) and 3_(S), there is a small difference in the change amounts ofthe respective detection voltages V_(2M), V_(2S). When the contactresistors R_(t) of both potentiometers 3_(M) and 3_(S) aresimultaneously increased at the same level, the value of .linevertsplit.ΔV_(M) -ΔV_(S) .linevert split. becomes zero. However, generallyspeaking, since there is substantially no probability that such acondition occurs, there is substantially no change amount in case of.linevert split.ΔV_(M) -ΔV_(S) .linevert split.≦β. In other words, it isrecognizable that the value of the contact resistor R_(t) is present ina no problem range, and the process operation is returned to the highorder main program. On the other hand, in case of .linevert split.ΔV_(M)-ΔV_(S) .linevert split.>β, the process operation is moved to a step S6in order to further judge as to whether or not it is under abnormalitystate.

At step S6, a comparison is made by checking whether or not such a valueobtained by subtracting the absolute value of the sub-sided torquevoltage change amount ΔV_(S) from the absolute value of the main-sidedtorque voltage change amount ΔV_(M) is greater than zero. Then, it is sojudged whether the main potentiometer, or the sub-potentiometer is undernormal state. This judgement formula is defined by the following formula(15):

    .linevert split.ΔV.sub.M .linevert split.-.linevert split.ΔV.sub.S .linevert split.≧0            (15)

As shown in FIG. 15, in conjunction with the increase of the contactresistor R_(t), the change amounts of the torque detection voltagesV_(2M) and V_(2S) are decreased. As a result, for instance, assumingthat the contact resistor R_(t) of the main potentiometer 3_(M) isincreased and there is no change in the contact resistor R_(t) of thesub-potentiometer 3_(S), since the change amount ΔV_(M) of themain-sided torque voltage is decreased, there is such a trend that thevalue of .linevert split.ΔV_(M) .linevert split.-.linevert split.ΔV_(S).linevert split. becomes a negative value. As described above, when.linevert split.ΔV_(M) .linevert split.-.linevert split.ΔV_(S) .linevertsplit.<0, it may be judged that the sub-potentiometer 3_(S) is undernormal state, as compared with the main potentiometer 3_(M). The processoperation is advanced to a step S7 in order to perform the subsequentprocess while using the sub-potentiometer 3_(S) as the reference.Conversely, when .linevert split.ΔV_(M) .linevert split.-ΔV_(S).linevert split.≦0, it may be judged that the main potentiometer 3_(M)is under normal state, as compared with the sub-potentiometer 3_(S). Theprocess operation is advanced to execute the subsequent process whileusing the main potentiometer 3_(M) as the reference.

At the step S7, a judgement is made as to whether or not it satisfies.linevert split.ΔV_(M) -ΔV_(S) .linevert split.≧(X/Y).linevertsplit.ΔV_(S) .linevert split., and another judgement is done as towhether or not there is a abnormality in the torque detection value ofthe main potentiometer, while using the sub-potentiometer 3_(S) as thereference. This judgement formula is calculated as follows. First, aninclined amount ΔG_(M) of the torque detection value V_(2M) with respectto the steering torque of the main potentiometer 3_(M) is calculatedbased on the below-mentioned formula (16):

    ΔG.sub.M =ΔV.sub.M / T(n)-T(n-1)!              (16),

where symbol "T(n)-T(n-1)" indicates a change amount of steering torqueevery preselected sampling time (timer interrupt time), and symbol "T"denotes input steering torque.

Similarly, an inclined amount ΔG_(S) of the torque detection voltageV_(2S) of the sub-potentiometer 3_(S) is calculated by the followingformula (17):

    ΔG.sub.S =ΔV.sub.S / T(n)-T(n-1)!              (17)

In the case that there is a abnormality in the torque detection valuedue to an increase in the contact resistor R_(t), the respectiveinclined amounts ΔG_(M) and ΔG_(S) are decreased. In this processoperation at the step S7, since it is assumed that the sub-potentiometer3_(S) is under normal state, a difference between the inclined amountΔG_(M) and the inclined amount ΔG_(S) is calculated while using theinclined amount ΔG_(S) as the reference. When this difference valueexceeds a preselected value X, it may be judged that the abnormalityscontained in the torque detection value of the main potentiometer _(M).This judgement formula is indicated by the following formula (18):

    .linevert split.ΔG.sub.M -ΔG.sub.S .linevert split.>X(18)

This predetermined value X is set to such a value slightly larger thanthe above-explained preset value β. An insensitive range is formedbetween the predetermined value β and a preselected value X, so that ajudgement about the normal/abnormal states can be performed under stablecondition.

When the formulae (16) and (17) are substituted for the formula (18),the below-mentioned formula (19) is given:

    .linevert split. ΔV.sub.M -ΔV.sub.S !/ T(n)-T(n-1)!.linevert split.>X                                                  (19)

Since the sub-potentiometer _(S) is under normal state, assuming nowthat the value of the contact resistor R1 is substantially equal tozero, the inclined amount ΔG_(S) is expressed by "KE" based upon theformula (11), and this incline amount is set as "Y", for the sake ofconvenience. Also, since it is difficult to directly calculate the valueof T(n)-T(n-1), the following calculation is carried out so as to cancelit.

In the formula (19), assuming now that ΔG_(S) =KE=Y in theabove-explained formula (17), when the term of T(n)-T(n-1) is canceled,the below-mentioned judgement formula (20) may be calculated:

    .linevert split.ΔV.sub.M -ΔV.sub.S ≧(X/Y).linevert split.ΔV.sub.S .linevert split.                     (20)

Since the inclined amount ΔG_(M) of the main potentiometer 3_(M) isdetected while using the sub-potentiometer _(S) under normal state inthis judgement formula, when it becomes .linevert split.ΔV_(M) -ΔV_(S).linevert split.<(X/Y).linevert split.ΔV_(S) .linevert split., it can bejudged that the torque detection value of the main potentiometer _(M) isunder normal condition. At this time, the process operation is returnedto the high order main program. On the other hand, when it becomes.linevert split.ΔV_(M) -ΔV_(S) .linevert split.≧(X/Y).linevertsplit.ΔV_(S) .linevert split., it can be judged that a abnormalityhappens to occur in the torque detection value of the main potentiometer3_(M). At this time, the process operation is advanced to a fail safeprocess defined at a step S8.

At the step S8, for instance, the below-mentioned process operation iscarried out as the fail safe process. First, the pulse width modulationsignal PWM, the right direction signal D_(R), and the left directionsignal D_(L), which are supplied to the motor drive circuit 22, are setto low levels, so that the supply of signals to the H bridge circuit 22bis stopped. Subsequently, the clutch control signal S_(a) is outputtedas a low-level, so that the electromagnetic clutch apparatus 11 isbrought into the non-connect condition. Then, the relay control signalS_(R) is outputted as a low level, so that the fail safe relay 23a isopened to stop the supply of the power supply voltage to the H bridgecircuit from the battery 16. Next, for example, the abnormality(abnormal) detection flag is set to "1", so that this occurrence of theabnormality is notified to the high order program, and the processoperation is ended. Subsequently, no interrupt process operation fordetecting the abnormality of the torque detection value is performed inthe high order program.

Then, at a step S9, a judgement is made as to whether or not it becomes.linevert split.ΔV_(M) -ΔV_(S) .linevert split.≧X/Y.linevertsplit.ΔV_(M) .linevert split.. Another judgement is done as to whetheror not there is a abnormality in the torque detection value of thesub-potentiometer _(S), while using the main potentiometer _(M) as thereference. This judgement formula may be obtained by executing acalculation sequence similar to that of the step S7. When it becomes.linevert split.ΔV_(M) -ΔV_(S) .linevert split.<(X/Y).linevertsplit.ΔV_(M) .linevert split., it can be judged that the torquedetection value of the sub-potentiometer 3_(S) is normal. In this case,the process operation is returned to the high order main program.

On the other hand, when it becomes .linevert split.ΔV_(M) -ΔV_(S).linevert split.≧(X/Y).linevert split.ΔV_(M) .linevert split., it can bejudged that a abnormality happens to occur in the torque detection valueof the sub-potentiometer _(S). At this time, the process operation isadvanced to the above-explained fail safe process defined at the stepS8.

Next, operations of the above-explained second embodiment will now bedescribed.

In accordance with the second embodiment, the abnormality detections ofthe torque detection value for the respective potentiometers 3_(M) and3_(S) is executed before every time the normal motor drive controlprocess operation is carried out. First of all, the respective changeamounts ΔV_(M) and ΔV_(S) of the respective torque detection voltagesV_(2M) and V_(2S) are calculated with respect to each of the interrupttimes. At the step S5, a judgement is made as to whether or not thevalue of .linevert split.ΔV_(M) -ΔV_(S) .linevert split. is smallerthan, or equal to the preselected value β, so that the abnormalityjudgement of the contact resistor R_(t), is simply carried out. In thecase of .linevert split.ΔV_(M) -ΔV_(S) .linevert split.≦β, it may bejudged that the contact resistor R_(t) is located within the normalrange, the normal motor drive control of the electric power steeringapparatus is executed.

On the other hand, when it becomes .linevert split.ΔV_(M) -ΔV_(S).linevert split.>β, the abnormality detecting process operation isfurther continued to thereby detect on which side of the mainpotentiometer _(M) and the sub-potentiometer _(S), the contact resistorR_(t) is increased. If it is so judged that the sub-potentiometer _(S)is under normal condition, another judgement is made at the step S7 asto whether or not the change amount ΔV_(M) of the torque detectionvoltage V_(2M) of the main potentiometer 3_(M), namely the decreaseamount is present within a preselected range, while using the changeamount ΔV_(S) of the torque detection voltage V_(2S) of thesub-potentiometer _(S) as a reference.

Then, when such a judgement is made that the main potentiometer 3_(M) isunder normal condition, a check is done a to whether or not the changeamount ΔV_(S) of the torque detection voltage V_(2S) of thesub-potentiometer _(S) is located within a preselected range. Inaccordance with this embodiment, although the judgement is made as towhether or not the change amounts ΔV_(M) and ΔV_(S) are present withinthe normal range also at the step S5, since the judgements are carriedout at the steps S7 and S9, while using the potentiometers 3_(M) and3_(S) having lower values of the contact resistors R_(t) as a reference,more precise abnormality judgement can be performed.

As a judgement result of the steps S7 and S8, when the increase of thecontact resistor R_(t) is located within the normal range, the normalmotor drive control of the electric power steering apparatus isperformed, whereas when the increase of the contact resistor R_(t)exceeds the normal range, a preselected fail safe process operation iscarried out to maintain the safety operation.

As previously explained, since the abnormality caused by the increase ofthe contact resistor R_(t) is detected based upon the main-sided torquevoltage change amount ΔV_(M) and the sub-sided torque change amountΔV_(S) in accordance with the second embodiment, it is possible todetect the relative change in the torque detection voltages V_(2M) andV_(2S) irrelevant to the magnitudes of the applied steering torque. As aconsequence, even when the very low steering force is produced as in theposition near the neutral position where there is no large change in thesteering torque, it is possible to detect the abnormality of the torquedetection value as same as when the steering torque is largely changed.It is easily possible to detect the abnormalitys contained in the torquedetection voltages V_(2M) and V_(2S), which are caused by the agingeffects of the contact resistance values of the sliding contactors andalso of the signal lines for the main potentiometer _(M) and thesub-potentiometer 3_(S).

Also, since the abnormality of the contact resistor R_(t) is relativelydetected based upon the main-sided torque voltage change amount ΔV_(M)and the sub-sided torque voltage change amount ΔV_(S), the settingvalues used as the reference to detect the abnormality can be determinedwithout considering the contact resistance values occurred while theconnector units of the cable 26 for connecting the torque sensor 3 withthe controller 13 are manufactured/assembled. As a consequence, there isno need to make up a larger set value, and it is possible to improve thedetecting precision. Even when the contact resistance values areincreased due to the aging effect, the abnormality of the torquedetection value can be firmly detected.

Referring now to FIG. 17, a circuit arrangement of a third embodimentaccording to the present invention will be explained.

This third embodiment has a similar arrangement as the torque sensor ofFIG. 12 except that in the above-described second embodiment, therespective ends of the main potentiometer _(M) and the sub-potentiometer_(S) of the torque sensor 3 are constructed of the respective individualterminals, and are connected via the cable 26 to the controller 13. As aconsequence, the respective end portions of both edges of the mainpotentiometer _(M) are connected to a current input terminal 3a and adrift output terminal 3c, respectively, and are connected via the cable26 to current detecting resistors 13d, 13g, respectively, and furtherare connected to the power supply and the ground potential. Then, therespective end portions of both edges of the sub-potentiometer 3_(S) areconnected to a current input terminal 3a' and a drift output terminal3c', respectively, and are connected via the cable 26 to currentdetecting resistors 13d, 13g respectively. It should be noted thatsliding contactors 3d and 3d' are connected in a similar manner to thatof the first embodiment, and torque detection voltages V_(2M) and V_(2S)derived from the sliding contactors 3d and 3d' are supplied to phasecompensating circuits 18 and 18'.

In accordance with this third embodiment, when there is such a notnegligible contact restor R_(c) between the current input terminals 3a,3a' and the power supply, an equivalent circuit, for example, as to themain potentiometer _(M) may be expressed as in FIG. 18. The torquedetection voltage V_(2M) in this equivalent circuit is expressed by thefollowing formula (21):

    V.sub.2M = R.sub.3 /(R.sub.1 +R.sub.3 +R.sub.c)!E          (21)

Then, in this formula (219, similar to the above-explained secondembodiment, it is assumed that R₃ /(R₁ +R₃)=α, the torque detectionvoltage V_(2M) may be represented by the following formula (22):

    V.sub.2M = α/(1+αR.sub.c /R.sub.3)!E           (22)

Further, in this formula (22), similar to the second embodiment, it isassumed that α=K·T+1/2, the torque detection voltage V_(2M) may beexpressed by the following formulae (23) and (24):

    V.sub.2M =(KE/γ)T+E/2γ                         (23)

note that,

    γ=1+(KT+1/2) (R.sub.c /R.sub.3)                      (24).

Therefore, when the contact resistor R_(c) is varied, since the value ofthe torque detection voltage V_(2M) is changed in response to the inputsteering torque T, the abnormality of the torque detection value causedby the increase in the contact resistor R_(c) can be detected bydetecting the change amount in the torque detection voltage V_(2M) everypreselected time. It should be understood that similar to the abovecase, the abnormality of the torque detection voltage caused by theincrease in the contact resistance value can be detected as to a contactresistor existing between the drift output terminals 3_(c), 3_(c') andthe ground potential.

As described above, in accordance with the third embodiment, there isthe effects achieved in the second embodiment, and furthermore, theincrease in the contact resistance value produced in the power supplyline can be detected. As a consequence, it is possible to detect thechanges in all of the contact resistance values existing between thetorque sensor 3 and the controller 13, and also possible to detect theabnormalitys of the torque detection values caused by theabove-described reasons. Therefore, a further improvement of safetycontrol operation may be achieved.

On the other hand, the drift detection process operation is carried outin accordance with the same manner as the first embodiment. A processoperation similar to that of the flow charts shown in FIG. 7, FIG. 8,and FIG. 10 is executed. The self steer of the steering wheel can beprevented by checking as to whether or not the drift voltage V_(is)appearing at the current detecting resistor 13g is present within thenormal range. When this drift voltage V_(is) is present within thenormal range, the normal motor drive control is performed. It shouldalso be noted that the resistance value of the current detectingresistor is made smaller than the resistance value of the embodimentwith the basic structure in order that both of the minimum value V_(min)and the maximum value V_(max) in the drift detecting process can beemployed as the same values.

Then, when no steering torque is applied, the torque is applied, thetorque detection values T₁ and T₂ become the same voltage values as thevoltages of the sliding contacts 3d₁, and 3d₂ at the neutral positionsclue to the effects of the torque detection value setting resistors13e₁, 13f₁ and 13e₂, 13f₂. As a result, at this time, even when theresistance values of the respective connector contact resistors at theinput/output terminals of the torque signal are increased to produce thedrift, the torque voltage V_(t) is maintained at constant value, so thatthe self steer of the steering wheel can be prevented.

Although the above-explained second and third embodiments have describedthe torque sensor 3 in which the main potentiometer _(M) and thesub-potentiometer _(S) have been assembled in one body, the presentinvention is not limited thereto. Alternatively, the respectivepotentiometers may be separately employed and may be located atdifferent positions.

In the above-explained second and third embodiments, the torquedetection voltage is detected from the torque sensor equipped with thepotentiometer, but the present invention is not limited thereto.Alternatively, two sets of main torque sensor and sub-torque sensorconstructed of a bridge arrangement partially having a coil may beprepared to detect torque detection voltages therefrom. A differencevalue is calculated from a change amount of each torque detectionvoltage every preselected time, so that a change in contact resistorsproduced in the connector unit and the like may be detected.

Also, in the second and third embodiments, the abnormality of the torquedetection value caused by the increase in the contact resistance valueshas been explained. Alternatively, when the respective potentiometers3_(M) and 3_(S) are disconnected, or shortcircuited, it may appear asthe abnormality of the torque detection value. Therefore, it is alsopossible to detect the abnormality of the torque detection value whenthe signal line is disconnected and/or shortcircuited in accordance witha flow chart shown in FIG. 16.

In the above-described second and third embodiments, when the formula(20) is modified, it is given as the following formula (25):

    .linevert split.ΔV.sub.M /ΔV.sub.S .linevert split.≧(X/Y)+1                                     (25)

Assuming now that the right hand of this formula (25) is equal to, forexample, a predetermined value "γ", the abnormality of the torquedetection value may be detected by comparing the predetermined value "γ"with the value of .linevert split.ΔV_(M) /ΔV_(S) .linevert split..

Although the abnormality judgements have been performed twice at thestep S5 and the steps S7, S9 in the above-described second and thirdembodiments. When no detection is performed which potentiometer _(M) or3_(S) is brought into the abnormality state, the process operationsdefined at the steps S6, S7, S9 may be omitted. In this case, at thestep S5, the predetermined value βs set to be such a value slightlylarger than this value. When it becomes .linevert split.ΔV_(M) -ΔV_(S).linevert split.>β, it may be judged that there is a abnormality in thetorque detection value. Then, the process operation is advanced to thestep S8 at which the fail safe process is carried out.

Although the torque detection value setting resistors 13e and 13f havethe same resistance values in the second and third embodiments, thepresent invention is not limited thereto. Alternatively, the torquedetection value setting resistors 13e and 13f having differentresistance values from each other may be arranged in such a manner thatthe output voltage of the potentiometer becomes equal to the torquedetection value supplied to the phase compensating circuit in responseto the output voltage of this potentiometer when the steering torquebecomes zero. This may be similarly applied to the other torquedetection value setting resistors 13e' and 13f '.

Furthermore, in the first to third embodiment, the motor drive signalS_(M) outputted from the central processing unit is converted into thepulse width modulation signal PWM to thereby drive the electric motor.Alternatively, the motor drive signal S_(M) is not converted into thepulse width modulation signal PWM, but into an analog voltage signal,and also NPN transistors may be employed instead of the FETs to whichthe pulse width modulation signals PWM are inputted. A voltageproportional to this analog voltage is applied to a base terminal ofeach of the NPN transistors in order to drive the electric motor.

Although all of the process operations, i.e, the proportional process,the differential process, and the integral process have been carried outto calculate the motor drive signal value in the above-explained firstto third embodiments. Alternatively, any of these process operations maybe arbitrarily combined to calculate the motor drive signal value.

Furthermore, the above-explained first to third embodiments haveexplained such a motor drive control that the steering condition issensed based on only the torque detection value, and the auxiliarysteering torque is produced in accordance with this torque detectionvalue. In other case, for example, when a drive lane is changed duringhighspeed drive, the steering conditions are sensed based upon not onlythe steering torque, but also the steering angular speed or the steeringacceleration speed of the steering wheel. Auxiliary torque in responseto these values may be produced to thereby perform the motor drivecontrol. In FIG. 19, there is shown a schematic block diagram of acontrol circuit for sensing a steering condition based on a torquedetection value, a steering angle speed value, and a steeringacceleration speed value.

As indicated in this drawing, a control circuit 21A is arranged by acurrent instruction calculator 31, an adder/subtracter 32, aproportional calculator 33, an integral calculator 34, an adder 35, asteering angular speed/acceleration speed calculating circuit 36, adamper coefficient circuit 37, and an inertia compensating coefficientcircuit 38. The torque detection value T is entered into the currentinstruction calculator 31 of the control circuit 21A so as to beconverted into a predetermined motor current instruction value S₁.Thereafter, this predetermined motor current instruction value issupplied to the adder/subtracter 32. In addition to the motor currentinstruction value S₁, a current detection signal i_(M), a damper signalD₁, and an inertia signal K₁, corresponding to the respective outputsignals from the current detecting circuit 25, the damper coefficientcircuit 37, and the inertia compensating coefficient circuit 38 aresupplied to the adder/subtracter 32. With respect to the motor currentinstruction value S₁, the adder/subtracter 32 subtracts the currentdetection signal i_(M), subtracts the damper signal D₁, and adds theinertia signal K₁. In the proportional calculator 33 to which the outputsignal of the adder/subtracter 32 is supplied, a preselectedproportional gain is multiplied. The multiplied value is directlysupplied to the adder 35, and is similarly supplied to the adder 35 viathe integral calculator 34 for performing a predetermined integralprocess. Then, a predetermined motor drive signal is outputted from theadder 35 to the motor drive circuit 22. In the motor drive circuit 22,the pulse width modulation signal PWM having a predetermined pulse widthis outputted to the steering angular speed/acceleration speedcalculating circuit 36, and the motor drive current is supplied to theelectric motor 12. Then, a motor drive current value "i" is outputtedfrom the electric motor 12 to the steering angular speed/accelerationspeed calculating circuit 36 and the current detecting circuit 25. Inthe steering angular speed/acceleration speed calculating circuit 36, asteering angular speed "ω_(o) " calculated based on the entered pulsewidth modulation signal PWM and the entered motor current "i" isoutputted to the damper coefficient circuit 37, and a similarlycalculated steering acceleration speed "ω₁ " is outputted to the inertiacompensating coefficient circuit 38.

The calculations about the steering angular speed ω_(o) and the steeringacceleration speed ω₁ in the steering angular speed/acceleration speedcalculating circuit 36 are performed as follows. First, when a dutyratio D of the pulse width modulation signal PWM and the power supplyvoltage V_(BAT) are employed, an average voltage V applied to theelectric motor 12 may be expressed by the following formula:

    V=D·V.sub.BAT                                     (26)

When the electric motor 12 is rotated, a back electromotive force isproduced. Assuming now that a back electromotive force constant is"K_(T) ", a back electromotive force voltage produced in the electricmotor 12 becomes K_(T) ×ω_(o). Therefore, the average voltage V appliedto the electric motor 12 having a coil resistance R may also beexpressed by the following formula:

    V=K.sub.T ·ω.sub.o +R·i            (27)

Based upon the formulae (36) and (27), the steering angular speed ω_(o)may be obtained as follows:

    ω.sub.o =(D·V.sub.BAT -R·i)/K.sub.T(28)

The steering acceleration speed ω₁ is calculated by differentiating thisformula (28) by time "t".

The calculated steering angular speed ω_(o) is multiplied by apreselected damper coefficient K_(V) in the damper coefficient circuit37, and this multiplied value is subtracted from the motor currentinstruction value S₁ to perform the damper control, so that electricviscosity resistance is given the steering system so as to improve thesafety drive operation of the vehicle. Also, the calculated steeringacceleration speed ω₁ is multiplied by a predetermined inertiacompensating coefficient K_(G) in the inertia compensating coefficientcircuit 38, and this multiplied value is added to the motor currentinstruction value S₁ so as to execute the inertia compensating control.As a consequence, a delay in the motor response characteristic caused bythe motor inertia is compensated. Alternatively, the steeringacceleration speed ω₁ may be directly detected by the sensor. Also, forinstance, an angle value detected by an angle sensor mounted on themotor shaft may be differentiated by time "t" to firstly calculate thesteering angular speed ω_(o) and further to differentiate this steeringangular speed to obtain the steering acceleration speed ω₁.

Utilization for Industrial Field

As previously described, according to the claim 1 of the presentinvention, the power supply current is supplied to the torque detectingpotentiometer, the current flowing through the potentiometer is inputtedinto the current detecting means, and the output value of the currentdetecting means is compared with the preset value by the comparing meansto detect a drift produced in the potentiometer. Since the change in thecurrents is monitored by the current detecting means, the change in thedrifts can be directly detected. The present invention has such anadvantage that the drift can be firmly detected by the simple circuitarrangement without lowering the detection sensitivity.

Then, according to the claim 2 of the present invention, there isprovided the torque detection value setting means for setting the torquedetection value to be equal to the output voltage of the potentiometerwhen no steering torque is applied. As a result, even when theresistance value of the connector contact resistor of the input/outputterminal for the torque signal is increased and thus the drift happensto occur, the torque detection voltage is kept constant by effects ofthe torque detection value setting means when no steering torque isapplied. Thus, the self steer of the steering wheel can be prevented toimprove the safety drive.

According to the claim 3 of the present invention, since the torquedetection valve setting means is arranged by the torque detection valuesetting resistor connected to the torque signal output terminal of thetorque sensor and between both ends of the power supply, the torquedetection voltage when no steering torque is applied can be keptconstant with a simple arrangement.

Furthermore, according to the claim 4 of the present invention, theabnormality detecting means for detecting the abnormality of the torquedetection value is provided in which calculation is made of thedifference value between the change amount of the torque detection valueper unit hour, outputted from the main torque sensor, and the changeamount of the torque detection value per unit hour, outputted from thesub-potentiometer. Accordingly, it is possible to obtain the stabledifference value of the torque detection values irrelevant to themagnitudes of the applied steering torque. As a consequence, even when avery small steering force is applied, i.e., a very small torquedetection value, the abnormality of the torque detection value can bedetected in a similar manner when a large steering force is applied. Itis easily possible to detect the abnormality of the torque detectionvalue caused by the aging effects of the contact resistance valuesoccurred in the signal lines of the main torque sensor and thesub-torque sensor.

Moreover, in accordance with the claim 4 of the present invention, whenthe abnormality of the torque detection value is detected, since thedifference value between the torque detection values of the main torquesensor and the torque detection values of the sub-torque sensor is notdirectly used, such a set value used as a reference to detect theabnormality is no longer defined by considering the contact resistancevalues produced during the manufacturing/assembling operations as to thecontact resistors of the connector unit for electrically connecting thecontrol means with the main torque sensor and the sub-torque sensor. Asa result, a need to set a larger set value can be canceled to improvethe detection precision. Even when the contact resistance values areincreased due to the aging effects, there is such an effect that theabnormality of the torque detection value can be firmly detected.

Moreover, in accordance with the claim 4 of the present invention, theabnormality of the torque detection value is detected based upon therespective change amounts of the torque detection values per unit hour,derived from the main torque sensor and the sub-torque sersor, so thatthe abnormality of the torque detection value caused by the increases inthe contact resistance value can be detected. Additionally, there isanother effect that the abnormality of the torque detection value causedby the disconnection and the shortcircuit of the respective torquesensors can be simultaneously performed.

According to the invention of this claim 5, the external connectionterminal of the torque sensor can be constructed of three components,i.e., one pair of connection terminals and external connection terminalcan be made simple, and can be easily connected to the control means.

In accordance with the invention of this claim 6, in addition to theadvantage of the claim 4, it is possible to detect the increase in thecontact resistance value occurred in the power supply line. The changesin all of the contact resistance values existing in the torque sensorand the control means can be detected, and the abnormalitys of thetorque detection values caused by the above contact resistance valuechanged can be detected.

According to the invention as defined in claim 7, the drifts produced inthe potentiometers can be firmly detected. Also, for example, theabnormalitys of the torque detection values such as the failure of thetorque sensor can be correctly detected, which are caused by thevariation in the torque detection value due to the aging effect of theconnector contact resistance value, and by the loose contacts of thesliding contactors of the potentiometers irrelevant to such a very smallsteering force under which the torque detection value is low, and such alarge steering force.

What is claimed is:
 1. In a control apparatus of an electric powersteering apparatus comprising a torque sensor equipped with a torquedetecting potentiometer having a resistance body, both ends of whichbeing connected to a power supply, and a sliding contactor slid on theresistance body for outputting a torque detection value, said torquesensor detecting steering torque of a steering system; an electric motorfor producing a steering auxiliary force to the steering system; controlmeans for outputting a control signal used to control the electric motorin response to at least the torque detection value outputted from thetorque sensor; and drive means for driving the electric motor inresponse to the control signal of the control means, the controlapparatus comprising:current detecting means for detecting a powersupply current flowing between both ends of the potentiometer; and driftdetecting means for comparing an output value of the current detectingmeans with a preset value to thereby detect a drift occurred in thepotentiometer.
 2. A control apparatus of an electric power steeringapparatus as claimed in claim 1, further comprising torque detectionvalue setting means for setting the torque detection value, suppliedwhen no steering torque is applied, to the same value as a voltage valueoutputted from the potentiometer to thereby output the set torquedetection value to the control means, the torque detection value settingmeans being arranged between the same power supply and ground as thoseof the potentiometer.
 3. A control apparatus of an electric powersteering apparatus as claimed in claim 2 wherein:said torque sensor isarranged by a current input terminal and a drift output terminalconnected to both ends of the power supply; a potentiometer connectedbetween both of the current input terminal and the drift outputterminal; and a torque signal output terminal connected to the slidingcontactor of said potentiometer, and said torque detection value settingmeans is constructed of torque detection value setting resistors eachconnected between said torque signal output terminal and both ends ofsaid power supply.
 4. In a control apparatus of an electric powersteering apparatus including a torque sensor equipped with a torquedetecting potentiometer having a resistance body, both ends of whichbeing connected to a power supply, and a sliding contactor slid on theresistance body for outputting a torque detection value, the torquesensor detecting steering torque of a steering system; an electric motorfor producing a steering auxiliary force to the steering system; andcontrol means for controlling the steering auxiliary force produced bythe electric motor in response to at least the torque detection valueoutputted from the torque sensor; wherein:the torque sensor isconstructed of a main potentiometer and a sub-potentiometer; the controlmeans controls the steering auxiliary force produced by the electricmotor based upon at least a torque detection value of the mainpotentiometer; and the control apparatus further comprises abnormalitydetecting means for calculating a change amount per unit hour at thesame time for each of torque detection values of the main potentiometerand of the sub-potentiometer, and also for calculating a differencevalue between the calculated change amount of the main potentiometer andthe calculated change amount of the sub-potentiometer to thereby detectan abnormality of the torque detection value based on the differencevalue.
 5. A control apparatus of an electric power steering apparatus asclaimed in claim 4, wherein the torque sensor includes connectionterminals connected to both ends of the power supply; main potentiometerand sub-potentiometer connected in parallel between the connectionterminals; and a torque detection value output terminal connected tosliding contactors of both of the main and sub-potentiometers.
 6. Acontrol apparatus of an electric power steering apparatus as claimed inclaim 4, wherein said torque sensor includes two sets of connectionterminals connected in parallel between both ends of the power supply; amain potentiometer connected to one set of the connection terminals; asub-potentiometer connected to the other set of the connectionterminals; and torque detection value output terminals connected tosliding contactors of both of the main and sub-potentiometers.
 7. In acontrol apparatus of an electric power steering apparatus including atorque sensor equipped with a torque detecting potentiometer having aresistance body, both ends of which being connected to a power supply,and a sliding contactor slid on the resistance body for outputting atorque detection value, the torque sensor detecting steering torque of asteering system; an electric motor for producing a steering auxiliaryforce to the steering system; control means for outputting a controlsignal used to control the electric motor in response to at least thetorque detection value outputted from the torque sensor; and drive meansfor driving the electric motor in response to the control signal of thecontrol means, wherein:the torque sensor is constructed of a mainpotentiometer and a sub-potentiometer; the control means controls thesteering auxiliary force produced by the electric motor based upon atleast a torque detection value of the main potentiometer; and thecontrol apparatus further comprises abnormality detecting means forcalculating a change amount per unit hour at the same time for each oftorque detection values of the main potentiometer and of thesub-potentiometer, and also for calculating a difference value betweenthe calculated change amount of the main potentiometer and thecalculated change amount of the sub-potentiometer to thereby detect anabnormality of the torque detection value based on the difference value;current detecting means for detecting a power supply current flowingbetween both ends of the potentiometer; and drift detecting means forcomparing an output value of the current detecting means with a presetvalue to thereby detect a drift occurred in the potentiometer.